Electrophotographic reproduction process including removal of electroscopic particles from developed electrostatic image



United States Patent O 3,322,537 ELECTROPHOTOGRAPHHC REPRODUC- TIONPROCESS INCLUDING REMOV- AL OF ELECTROSCOPIC PARTICLES FROM DEVELOPEDELECTROSTATIC IMAGE Edward C. Giaimo, Jr., Princeton, NJ., assignor toRadio Corporation of America, a corporation of Delaware Filed Nov. 29,1963, Ser. No. 326366 8 Claims. (CI. 96-11) This invention relatesgenerally to the art of electrophotographic reproduction, and moreparticularly to an improved method of developing an electrostatic latentimage on :an electrophotographic record element. The improved method ofthe present invention is useful particlarly, but not ex-clusively, forproviding information, either temporarily or permanently, on a reusablerecord element.

It has been proposed to develop an electrostatic latent image on thesurface of a record element by a process wherein a toner composed ofelectroscopic particles is applied to the latent image and the recordelement is heated to melt or fuse the electroscopic toner into therecord element. Such :an electrophotographic process is described in anarticle, Electrofax," Direct Electrophotographic Printing on Paper, byC. J. Young and H. G. Greig, RCA Review, December 1954, Volume XV, No.4. It has also been proposed to produce .a surface-modulated orrippleimage of an electrostatic latent image on a thermoplastic layer of :arecord element by a method wherein the record element is heated tosoften the thermoplastic layer, and the electrostatic forces on thesoftened thermoplastic layer produce the ripple image therein. Such aprocess is described in an article, Thermoplastic Recording, by W. E.Glenn, Journal of Applied Physics, Volume 30, No. 12, December 1959.

While the aforementioned methods of developing images on record elementsare suitable for many practical applications, it is sometimes desrableto retain an image on .a record element f or a relatively short periodof time in a manner whereby the image may be viewed either directly orindirectly through an optical system. It may also be desirable topresent this information quickly and temporarily in a manner whereby itmay be erased easily so that the same record element may be reused topresent additional information.

It is an object of the present invention to provide an improved methodof developing an image on an electrophotographic record element so thatthe image may be viewed either directly or indirectly th-rough anoptical system.

Another object of the present invention is to provide an improved methodof electrophotographic developing wherein a developed image may beerased easily from a record element so that the record element may bereused.

Still another object of the present invention is to provide an improvedelectrophotographic method of quickly developing an image having acontinuous tone rendition.

A further object -of the present invention is to provide an improvedelectrophotographic method of the type described that is relatively easyto apply to known record elements, inexpensive to execute, and highlyefficient in USB.

Briey stated, the improved method of developing an electrostatc latentimage in accordance with the present invention is practiced inconnection with an electrophoto- -graphic record element of the typecomprising a thermoplastic layer, which preferably, although notnecessarily, has photoconductive properties. A toner of electroscopicparticles, either in the dry form or Suspended in a liquid carrier suchas silicone oil, for example, is applied to the thermoplastic layer torender the latent image visible. The meltng point of the electroscopictoner particles should be relatively higher than that of thethermoplastic layer. The toned record element is heated toa temperaturesufficient to soften the thermoplastic layer but insuificient to meltthe electroscopic particles. Under these conditions, depressions ordimples are formed in the thermoplastic layer directly beneath theelectroscopic particles, forming a depressed stippled image of thelatent image. The developed l-aten-t image may now be viewed directly onthe record element. If it is desired to project the developed image ontoa screen, the electroscopic particles may be removed from thethermoplastic layer by blowing or washing them oil, and the remainingdimpled image, having a stippled, frosted-like appearance, may beprojected onto a screen with a conventional type optical projector orwith a schlieren optical projector. If it is desired to reuse the recordelement for additional information, the record element may be heated to.a temperature to melt the thermoplastic layer so as to smooth it. Therecord element, when cooled, can then be reused in the manner describedin the aforementioned process.

The novel features of the present invention, both as to its organizationand method of operation, as well as additional objects and advantagesthereof, Will be more readily understood from the following description,when read in connection with the accompanying drawing, in which similarreference characters refer to similar parts throughout, and in which:

FIG. 1 is a fragmentary, cross-sectional view of one example of anelectrophotographic record element useful in the improved method of thepresent invention; and

FIGS. 2, 3, 4, 5 and 6 are fragmentary, cross-sectional Views of therecord element illustrated in FIG. -1 in different stages of theimproved electrophotographic developing method of the present invention.

Referring, now, particularly to FIG. 1 of the drawing, there is shown`an example of an electrophotographic record element 10 of the typeuseful in connection with the improved developing method of the presentinvention. The record element 10 comprises a flexible or rigid substrate12, preferably of transparent material, such as "Mylar" or glass, atransparent or refiective conductive coating 14, such as tin oxide oraluminum, and a thermoplastic layer 16. The thermoplastic layer 16 maycomprise a solid solution of a photoconductor and a thermoplastic resin.Suitable thermoplastic resins in the solid solution of the thermoplasticlayer 16 are:

(1) Chlorinated paraffins, such as Chlorowax 70, Diamond Alkali Co.,Cleveland, Ohio.

(2) Polyvinylidene chloride.

(3) Polyvinylidene chloride copolymers, such as Saran F-120, The DowChemical Company, Midland, Mich.

(4) Polyvinyl chloride.

(5) Vinyl chloride copolymers, such as Vinylite; VAGH, 91% Vinylchloride, 3% Vinyl acetate, and 6% Vinyl alcohol; VYCM, 91% vinylchloride and 9% vinyl acetate; VMCI-I, 86% vinyl chloride, 13% Vinylacetate, and 1% dibasic acid, Bakelte Division, Union Carbide andCarbon, New York 17, N.Y.

(6) Polystyrene.

(7) Styrene copolymers, such as Piccotex 120, Pennsylvania IndustrialChemical Company, Clairton, Pa.

(8) Butadiene copolymers, such as Pliolite S-SD, Goodyear Tire andRubber Co., Akron, Ohio.

(9) Polyacrylates, such as Acryloid B-72, Rohm and Haas Co.,Philadelphia, Pa.

(10) Acrylic copolymers, such as Lucite 46, E. I. du Pont de Nemours andCo., Wilmington, Del.

(ll) Epoxy resins, such as Epon 1002, Sheel Chemical Company, Houston,Tex.

(12) Thermoplastic hydrocarbon terpene resins, such as Piccolyte S-135,Pennsylvania Industrial Chemical Company.

Many advantageous results are also obtainable when combinations ofresinous materials are employed, for example, such as:

(13) Compatible mixtures of various resinous materials,

such as Acryloid B-72 and Chlorowax.

-cyl Y wherein R and Rg are selected from the class consisting ofmono-alkylamino, di-alkylamino, mono-arylamino, and alkylarylamino; X isselected from the class consisting of H, and

wherein R is selected from the class consisting of H, OH, CH OCH and Rand wherein R, and R are selected from the class consisting of H, OH, CHand OCH and Y is H except when X-l-Y is double bonded oxygen.

A photoconductor is not necessary in the thermoplastic layer 16 if theelectrostatic latent image is to be produced on the layer 16 by thedirect deposition of electric charges, as by directing electrons ontothe layer 16 through an apertured Stencil.

In carrying out the improved developing method of the present invention,the record element 10 is charged electrostatically by any suitable meansknown in the art. For example, the layer 16 may be rubbed with silk orfur to deposit a uniform electrostatic charge thereon. The layer 16 mayalso be charged by corona discharge devices of the type described in theaforernentioned article in the RCA Review. The outer surface of thelayer 16 may be charged negatively, as shown in FIG. 2, or positively.Where the layer 16 contains a photoconductor, the electrostatic chargeis deposited uniformly on the layer 16 in the dark.

An electrostatic latent image can be produced on the layer 16 byselectively discharging the charged layer 16, as by electromagneticradiation. For example, the charged layer 16 can be exposed to light,illustrated by arrows 18, through an apertured Stencil, or aphotographic transparency 20, such as a photographic negative orpositive, as shown in FIG. 3. The light-struck areas of the layer 16 aredischarged in proportion to the amount of electromagnetic radiation,that is, light, impinging on these areas.

The electrostatic latent image may be rendered visible, that is,developed, by applying to the surface of the exposed layer 16 a toner,dry or wet, of electroscopic particles 22, as shown in FIG. 4. Ifpositively charged toner particles 22 are used, they are attracted tothe negatively charged portions of the exposed layer 16 in an amountproportional to the intensity of the electrostatic charges thereon, thusproducing a visible image having a continuous tone rendition. The latentimage may be developed to provide a visible image that may be consideredthe negative image of the aforementioned printed image by applyingnegatively charged electroscopic toner particles 22 to the exposed layer16.

Some examples of suitable toners of electroscopic particles useful inthe method are:

Example I.-50 grams of polyvinyl chloride-acetate resin, for example,composed of polyvinyl chloride and 10% polyvinyl acetate, and 2 grams ofzine oxide are milled for about 4 hours with quartz pebbles in a smallporcelain ball mill. 0.4 part of the milled material are mixed withparts of about 0.015 inch diameter glass beads to form the developermix, a toner of dry electroscopic particles.

Example II.-200 parts by weight of Piccolastic Resin 4358A (an elasticthermoplastic resn composed of polymers of styrene, substituted styreneand its homologs of the Pennsylvania Industrial Chemical Corp.,Clairton, Pa.).

12 parts by weight carbon black.

12 parts by weight Nigrosine SSB-Color Index No. 50415.

8 parts by weight Iosol Black- Color Index Solvent Black 13.

This developer material is prepared by melting the resn and mixing inthe other materials. When a uniform mix is obtained, it is cooled,ground to a fine powder and Classified to obtain a desired particlesize. A convenient particle size is one obtained by screening through a200 mesh which provides a maximum particle diameter of about 74 microns.This developer material may be dispersed in liquid by any of thecommonly known techniques to form a wet toner of electroscopicparticles.

Example III.--Toners of colored electroscopic particles, consisting ofOrganic pigments dispersed in dimethyl polysiloxane liquid may be used,prepared as follo-ws: Preferably, the dispersions comprise up to about20 parts by weight of pigment, the remainder being liquid. The termpigment, as em-ployed herein, is intended to include coloring agentswhich are sometimes referred to as dyes but which nevertheless areinsoluble in the polysiloxane. Suitable pigments for such purposesinclude the following:

(1) Cyan Blue Toner GT (described in U.S. Patent 2,486,351 to Richard H.Wiswall, Jr.).

(2) Benzidine Yellow (Color Index No. 21090).

(3) Brilliant Oil Blue BMA-Color Index No. 61555.

(4) Sudan 3 Red-Color Index No. 26100.

(5) Oil Yellow ZG-Color Index No. 11020.

(6) Pyrazolone pigment (such as Color Index No.

21080-Color Index Pigment Red 39).

(7) Hansa Yellow G-Color Index No. 11680.

In many of the foregoing dispersions it is convenient to provide asurfactant (surface active agent) to enhance the electrical propertiesof a selected pigment. A surfactant solution may be prepared bydispersing 10 gr ms of Nalcamine G-14 in 20 grams of toluene and, whilemixing, heating the dispersion to dissolve the Nalcamine G-l4- in thetoluene. Nalcamine G-l4 is a chemical of the type 1-(2-hydroxyethyl)2-hydrogenated tallow-Z- imidazoline (National Aluminate Corp., Chicago,Ill.). The Nalcamine G-l4 solution is added to the pigment dispersions'before they are ball milled in the dimethyl polysiloxane. Such ,asurfactant when applied, for example, to a red pyrazolone pgmentsubstantially enhances the electropositive nature thereof.

The method of developing described herein is applicable to toned unfixedimages on a thermoplastic layer, regardless of the process of depositingthe image of electroscopic particles on the thermoplastic layer. Forexample, a toned powder image may be transferred to the thermoplasticlayer 16 of the record element by any transfer process known in the art.The powder image prodnced on one surface of a record element by anyknown pror art method may be transferred to the thermoplastic layer 16by placing the thermoplastic layer 16 in contact with the powder image.and applying a charge of tappropriate polarity, as With a coronadischarge device, to the thermoplastic layer to attract theelectroscopic powder thereto. The transferred powder image on thethermoplastic layer 16 may now be worked upon, as will hereinafter bedescribed.

A toned powder image may also be produced on the thermoplastic layer 16by first transferring an electrostatic latent image from one recordelement to the record element 10. In this method, one record elementWith an electrostatic latent image thereon is placed against thethermoplastic layer 16 of the record element 10, and the latent image istransferred to the record element 10 by applying an appropriate chargeto the latter of the proper polarity. The electrostatic latent image onthe thermoplastic layer 16 may now be rendered visible by applying asuitable toner of electroscopic particles to it.

Where the developed unfixed image is of temporary significance, it maybe viewed in the form illustrated in FIG. 4 of the drawing.

It is sometimes desirable to use a relatively small record element 10and to enlarge the developed image thereon, as by projecting the imageonto a screen through an optical system. For this purpose, the recordelement 10 with its visible image of toner particles 22 is heated,either on .a hot plate, or by radiant heat, or by a stream of hot gas,illustrated by arrows 24, to a temperature at which only thethermoplastic layer 16 softens. The electroscopic toner particles 22should have a relatively higher melting point than the thermoplasticlayer 16. Accordingly, the layer 16 should be heated to `a temperaturesufficient to soften it only, but insuflicient to soften the tonerparticles 22. Under these conditions, depressions -or dimples 26 areformed in the surface of the layer 16 directly beneath the tonerparticles 22, as shown in FIG. 5. The dimples 26 provide a stippled,frosted-like visible image of the electrostatic latent image. The recordelement 10, as shown in FIG. 5, may now be viewed through any suitableoptical system (not shown) since the substrate 12, the conductivecoating 14, and the thermoplastic layer 16 are transparent, except forthe developed image.

Suitable record elements for practicing the improved developing methodof the present invention may be prepared in acoordance with thefollowing examples:

Example A A thermoplastic layer for the record element 'consists of thefollowing ingredients:

The aforementioned ingredients are mixed and heated to about 120 C. toform a solution that is applied to a suitable substrate, such as, forexample, a conductive glass or a metallized transparent pl astic. Apreferred substrate is lantern slide glass having on one surface thereofa vacuum deposted cond uctive film such as, for example,

tin chloride. When cooled, the thermoplastic layer of the record elementhas a melting point in the neighborhood of -100 C.

Example B The following ingredients for a thermoplastic layer of arecord element are combined by weight percent:

31.4% bis(4,4' dimethylaminophenyl)-z-hydroxyphenol methane 4,4' [2hydroxybenzylidene bis(N,N dimethylaniline) 672% sucrose benzoate, and

1.4% Vinylite VAGH, a copolymer of 81% vinyl chloride, 3% vinyl acetate,and 6% vinyl alcohol made by Bakelite Division of Union Carbide andCarbon, New York 17, N.Y.

The above ingredients, constituting the composition of the desired solidsolution of the thermoplastc layer, are dissolved in a solventconsisting of 21% methyl ethyl ketone and 32% toluene, the percent ofthe solvents being in weight percent of the total composition. Thesolution is applied to the surface of a glass lantern slide metallizedwith tin chlorde and dried by gentle heating at about 40 C. When cooled,the resulting thermoplastic layer has a meltng point at around 70 C.

Example C A thermoplastic layer comprises the following ingredients byweight percent:

332% bis(4,4'-dimethylaminophenyl)-phenyl methane 4,4'- benzylidene-bis(N,N-diethylaniline) 65.5 sucrose benzoate, and

13% Vinylite VAGH.

When these ingredients are thoroughly mixed, 48% by weight of themixture is mixed with 20% by weight of methyl ethyl ketone and 32% byweight of toluene, the percent weight of the solvents being in weightpercent of the total composition. The materials of the thermoplasticlayer are dissolved in the solvent and the solution is applied to themetallized coating (tin chloride) on a glass lantern slide as by rollcoating, flow coating, or dipping. When the thermoplastic layer isdried, it has a melting point in the neighborhood of 70 C.

If desired, the electroscopic toner particles 22 may be removed as byblowing them or by washing them ofi the layer 16, leaving a depressedstippled image of the latent image, as shown in FIG. 6. The removal ofthe toner particles may be aided by dischargng the thermoplastic layer16 completely, as by exposing it to bright light. The stippled image maybe projected upon a screen by either a conventional or a schlierenoptical system. The record element 10 may also be stored in thiscondition, the stippled image providing a permanent record. Where thisrecord is merely of temporary importance, the dimpled image may beremoved from the layer 16 by applying heat to the layer 16 of anintensity sufficient to melt it'so as to smooth it, thereby to erase thedimpled image thereon. Thus, the record element 10 may be reused in themanner heretofore described.

The record element 10 with its depressed stippled image formed by thedirnples 26 may be used as a printing plate in an intaglio printingpress. When engraving ink is deposited in the depressed stippled image,the ink may be transferred to a sheet of paper by pressing the recordelement 10 against the paper sheet.

From the foregoing description, it will be apparent that there has beenprovided an improved method of printing information on anelectrophotographic record element in a manner whereby the informationmay be retained permanently or temporarily, and also in a manner wherebythe printed image may be viewed directly or indirectly through anoptical system. It is also apparent that the improved method of thepresent invention provides an economical use of record elements in thatthe record elements may be reused repeatedly and the electroscopic tonerparticles may be reclaimed. While only a limited number of examples ofapplications of the improved printing method of the present inventionhave been described, variati-ons coming within the spirit of thisinvention Will, no doubt, readily suggest themselves to those skilled inthe art. Hence, it is desired that the foregoing shall be eonsidered asillustrative and not in a limiting sense.

What is claimed is:

1. A method of producing an image, said method comprising the sequentialsteps of (a) charging a thermoplastic photoconductive layer With anelectrostatic charge,

(b) exposing said charged layer to an image of electromagnetic radiationto discharge said layer selectively, whereby to produce an electrostaticlatent image thereon,

(c) adding a toner comprising electroscopic particles to said exposedlayer to render said latent image visible, the softening temperature ofsaid particles being higher than the softening temperature of saidlayer,

(d) applying heat to said layer to soften said layer only, said heatbeing of a temperature sufficient to soften said layer but insufficientto melt said particles, whereby depressions are formed in said layerbeneath said particles, and

(e) removing said particles from said layer, whereby to leave an imagecomprising depressions in said layer.

2. A method of producing an image comprising a plurality of depressionsin a thermoplastic photoconductive layer of a record element, saidmethod comprising the sequential steps of (a) charging said layer with auniform electrostatic charge,

(b) exposing said charged layer to a light image to discharge said layerselectively, whereby to form an electrostatic latent image thereon,

(c) adding a toner comprising electroscopic particles to said exposedlayer to render said latent image visible, the softening temperature ofsaid particles being higher than the softening temperature of saidlayer,

(d) directing a stream of hot gas onto said layer to soften said layeronly, said stream of hot gas being of a temperature sufficient to softensaid layer but insufficient to melt said toner, whereby said depressionsare formed in said layer beneath said particles, and

(e) removing said particles from said layer, whereby to leave said imagecomprising said depressions in said layer. i

3. A method of producing an image comprising a plurality of dimples in athermoplastic photoconductive layer of a record element, said methodcomprising the sequential steps of (a) exposing said layer to an imageof electromagnetic radiation to charge the conductivity of said layerselectively,

(b) charging said layer with an electrostatic charge to produce thereonan electrostatic latent image of said image of electromagneticradiation,

(c) adding a toner comprising electroscopic particles to said exposedand charged layer to render said latent image thereon Visible, thesoftening temperature of said particles being higher than the softeningtemperature of said layer,

(d) applying heat to said layer to soften said layer only, said heatbeing of a temperature sufficient to soften said layer but insufficientto melt said toner, whereby said plurality of dimples are formed in saidlayer beneath said toner particles,

(e) removing said particles from said layer, whereby to leave said imagecomprising said dimples in said layer, and

(f) heating said layer sufficently to cause said dimples to disappear,whereby said record element may be reused.

4. In an electrophotographic method wherein an electrostatic latentimage is produced on a thermoplastic layer of a record element, theimprovement comprising the sequential steps of (a) adding a tonercomprising electroscopic particles to said layer to render said latentimage visible, said toner having a melting point that is higher thanthat of said layer,

(b) applying heat to said layer, said heat being of an intensitysufficient to soften said layer but insumcient to soften said particles,whereby to form depressions in said layer directly beneath saidparticles, and

(c) removing said particles from said layer, whereby to leave an imagecomprising said depressions in said layer.

5. In an electrophotographic method wherein an electrostatic latentimage is produced on a thermoplastic layer of a record element, theimprovement comprising the sequential steps of (a) adding a tonercomprising electroscopic particles to said layer to render said latentimage visible, said toner having a softening temperature higher thanthat of said layer,

(b) directing a stream of hot gas onto said layer, said stream of hotgas having a temperature suflicient to soften said layer butinsufficient to soften said particles, whereby to form dimples in saidlayer directly beneath said particles,

(c) removing said particles from said layer, whereby to leave an imagecomprising said dimples therein, and

(d) heating said layer to a temperature sufiicient to melt said layer,whereby to remove said dimples and to smooth said layer so that it maybe reused when desired.

6. In an electrophotographic method of the type Wherein an image ofelectroscopic particles is produced on a thermoplastic layer of a recordelement, and wherein said particles have a higher melting point thansaid layer, the improvement comprising the sequential steps of (a)applying heat of a predetermined temperature, said predeterminedtemperature being sufficient to soften only said layer but insufficientto soften said particles, whereby to form depressions in said layerdirectly beneath said particles, and

(b) removing said particles from said layer.

7. In an electrophotographic method of the type Wherein an image ofelectroscopic toner particles is produced on a thermoplastic layer of arecord element, the improvement comprising the sequential steps of (a)directing a stream of heated gas onto said layer, said particles havinga higher melting point than said layer, and said stream of heated gashaving a temperature sufficient to soften said layer but insufficient tosoften said particles whereby to form dimples in said layer directlybeneath said particles,

(b) cooling said layer, and

(c) removing said particles from said cooled layer, whereby to leave arimage comprising said dimples in said layer.

8. In an electrophotographic method of the type Wherein an image ofelectroscopic toner particles is produced on a thermoplastic layer of arecord element, the improvemerit comprising the sequential steps of (a)heating said layer, said particles having a higher melting point thansaid layer, said heat having a temperature suflicient to sotten saidlayer but insufiicient to soften said particles, whereby to form dimplesin said layer directly beneath said particles,

and (b) cooling said layer, (c) removing said particles from said cooledlayer, whereby to leave an image comprising said dimples in said layer,and

(d) reheatng said layer to smooth the surface of said layer, wherebysaid record element may be reused.

References Cited UNITED STATES PATENTS Sugarrnan 96-1 Mast et al 88-61Lauriello 96-1.8 Dreyfoos et al 96-1 X 10 Greig 96-1 Ebert 96-1 Giamo96-1 Metcalfe et al. 96-1.5 15

l 0 OTHER REFERENCES Claus, Advances in Xerography," Phot. Sc. and Eng.,vol. 7, No. 1, January 1963, pp. 11 and 12.

Gundlach et al., "A Cyclic Xerographic Method Based on Frost Deformaton,Phot. Sc. and Eng., vol. 7, No. 1, January 1963, pp. 14-19.

Sugatan, Japanese Pat. SHO-37-4484, Mar. 21, 1960, 1 page of spec.;drawing on same page.

Olin, Photoplastic Recording," Industrial and Engineering Chemistry,vol. 55, No. 6, June 1963, pp. 11 and 12.

NORMAN G. TORCHIN, Primary Exam'ner. A. LIBERMAN, C. E. VAN HORN,Assistant Exam'ners.

1. A METHOD OF PRODUCING AN IMAGE, SAID METHOD COMPRISING THE SEQUENTILASTEPS OF (A) CHARGING A THERMOPLASTIC PHOTOCONDUCTIVE LAYER WITH ANELECTROSTATIC CHARGE, (B) EXPOSING SAID CHARGED LAYER TO AN IMAGE OFELECTROMAGNETIC RADIATION TO DISCHARGE SAID LAYER SELECTIVELY, WHEREBYTO PRODUCE AN ELECTROSTATIC LATENT IMAGE THEREON, (C) ADDING A TONERCOMPRISING ELECTROSCOPIC PARTICLES TO SAID EXPOSED LAYER TO RENDER SAIDLATENT IMAGE VISIBLE, THE SOFTENING TEMPERATURE OF SAID PARTICLES BEINGHIGHER THAN THE SOFTENING TEMPERATURE OF SAID LAYER, (D) APPLYING HEATTO SAID LAYER TO SOFTEN SAID LAYER ONLY, SAID HEAT BEING OF ATEMPERATURE SUFFICIENT TO SOFTEN SAID LAYER BUT INSUFFICIENT TO MELTSAID PARTICLES, WHEREBY DEPRESSIONS ARE FORMED IN SAID LAYER BENEATHSAID PARTICLES, AND (E) REMOVING SAID PARTICLES FROM SAID LAYER, WHEREBYTO LEAVE AN IMAGE COMPRISING DEPRESSIONS IN SAID LAYER.